The Mariner’s Astrolabe

The mariner’s astrolabe was an instrument used to measure the altitude of a celestial body above the horizon. The earliest recorded uses of the astrolabe for navigational purposes are by the Portuguese explorers Diogo de Azambuja (1481), Bartholomew Diaz (1487/88) and Vasco da Gama (1497/98) during their sea voyages around Africa. The Astrolabe was a simple brass ring, graduated in degrees with a rotating alidade for sighting the Sun or a star. The ring was cast brass, quite heavy and cut away to keep it from blowing around in the wind. It was not a very good instrument and errors of four or five degrees were common. (See Pic)

The Mariner’s Astrolabe was used to determine the latitude of a ship at sea by measuring the noon altitude of the Sun or the meridian altitude of a star of known declination. It was impossible to determine longitude at sea for several more centuries. To go to a place of known latitude, the ship was sailed to that latitude and then sailed east or west along the latitude line until the place was reached. To find the latitude of the ship at sea, the noon altitude of the Sun was measured during the day or the altitude of a star of known declination was measured when it was on the meridian (due north or south) at night. The Sun’s or star’s declination for the date was looked up in an almanac. The latitude is then 90° – measured altitude + declination . Tomorrow, another invention. Rita Bay

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